1. Field
This invention relates to a catheter instrument which measures physiological parameters of blood while same is located inside the human blood stream. The technique is made feasible by means of an elongated optical fiber transducer which in a well-known manner transmits light into the blood and carries the reflectance of that light back to the instrument from which it was transmitted.
2. State of the Art:
Devices for performing such measurements are known as oximeters and same are disclosed in the Shaw U.S. Pat. Nos. 3,638,640; 3,847,483; 4,114,604; 4,295,470; 4,416,285; 4,322,164 and Vurek U.S. Pat. No. 3,799,672 and Heinenmann U.S. Pat. No. 4,447,150 among others. These patents deal primarily with measurement of oxygen saturation in the blood. Oxygen saturation is the relative amount of oxygenated hemoglobin in all of the hemoglobin of the blood stream. Hemoglobin is packed in bioconcave disks of shaped red blood cells having a diameter of approximately 10 micrometers. Whole blood has a density of about 5 million red blood cells per cubic millimeter. Since the red blood cells both scatter and transmit the incident radiant energy, the differential absorption by oxygenated and non-oxygenated hemoglobin of the radiant energy transmitted through the blood gives a basis for oxygen saturation measurement. It can be seen that an optical fiber catheter transmits light to the position of interest within the flowing blood stream and a return fiber optic light guide conducts the reflected light from the blood stream back to a photo detector.
When blood in a human body is the test medium, there are a number of problems with measuring oxygen saturation which arise. These problems are fully detailed in the aforesaid patents. Briefly, however, the transducer itself introduces errors due to the two fiber optics connected to the detector system used to measure the light transmitted and reflected. In addition to this, the blood flow is pulsatile and as such the conditions to be measured are constantly fluctuating. Previous mathematical compensations for changes in hematocrit blood flow velocity, pH, PC02, and the like introduce errors into the oxygen saturation measurement. Similarly, variations in osmolarity and in transmissivity of the two optical fibers is also present and can result in influencing the ultimate measurement.
Several wavelengths are necessary in order to make measurement. That is to say that, light must be transmitted to the oxygenated hemoglobin at a minimum of two different wavelengths and the reflectance of those wavelengths when compared with the light transmitted gives the oxygen saturation in accordance with the following equation: ##EQU1## As explained in the Shaw U.S. Pat. No. 4,114,604, oxygen saturation is a function of the ratios of light intensity measurement of the several wavelengths.